


Praying for Mercie

by tansybells



Series: Succubus and Saint [1]
Category: Fire Emblem: Fuukasetsugetsu | Fire Emblem: Three Houses
Genre: Alternate Universe - Succubi & Incubi, Corruption, F/F, Female My Unit | Byleth, Mercedes is a Beautiful Fat Lady, Mercie is Super Into the Goddess, Sex Magic, Succubus Byleth, Top Mercedes von Martritz
Language: English
Status: Completed
Published: 2020-07-30
Updated: 2020-07-30
Packaged: 2021-03-05 19:27:58
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,667
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/25600591
Author URL: https://archiveofourown.org/users/tansybells/pseuds/tansybells
Summary: The demon Byleth is sent forth to corrupt an exceptionally holy woman, a devout follower of the Goddess. Yet when she actually approaches Mercedes, she finds that doing so is easier said than done.Succubus Byleth/Church Girl Mercedes PWP
Relationships: Mercedes von Martritz/My Unit | Byleth
Series: Succubus and Saint [1]
Series URL: https://archiveofourown.org/series/1977550
Comments: 13
Kudos: 83





	Praying for Mercie

As always, the trip from hell to the mortal world is an easy one. It’s a  _ lot  _ easier to go the other way, obviously, but it only takes a single step through a carefully constructed portal for the demon Byleth to find herself in a small, modest room. Only a cross hanging on the wall, illuminated by a small glowing rock on a bedside table, indicates to her that she is where she’s supposed to be: the home of one of the most pious people to have ever walked the earth.

Looking around, she notes that there’s little else decorating the room. Her prey for the night seems to have fully embraced the church’s tendency to forgo materialism. While it does make the room impossibly drab, it does mean that it’s easy for Byleth to hone in on the person she’s here to seduce.

There’s a blanket-wrapped figure upon the full-sized bed, which is to be expected. Readying herself to do her job—and do it well—Byleth stalks across the room and gently pulls the blanket off of the woman sleeping beneath. She’s slow enough, delicate enough, that she manages to do so without waking the human, and as soon as the blanket is discarded to the side, Byleth lifts herself up onto the bed alongside her.

Straddling the woman’s lower abdomen and leaning forward, Byleth props her elbows up on the mattress on either side of her victim’s head. Cocking her head to the side, she looks over the woman’s face and smiles deviously.

When the Big Guy had told her that she was gonna get shunted up to the ground floor to go corrupt an exceptionally holy woman, Byleth had originally assumed that she was going to have to somehow get it on with a wrinkly old woman. Well. It wouldn’t have been  _ hard,  _ since being a succubus made it impossibly easy to get it on with anyone, but that didn’t mean that boning an old lady wasn’t  _ disgusting. _

To her delight, however, she’s sitting above one of the most beautiful ‘holy people’ she’s ever seen.

Long, dreamer’s lashes flutter on her gently freckled cheeks, which are plump and beautifully curved thanks to eating well in a time of plenty. Her bobbed blonde hair is splayed out on the pillow around her, but if Byleth had to guess how it’d look when she’s awake, she’d say it wouldn’t go much further than her chin. She’s gorgeous in a serene, mature way, and if Byleth had any need of air, she has the feeling that she’d be left absolutely breathless.

The woman, who she had assumed to be asleep, now looks at her with blue eyes so clear that Byleth has a difficult time believing that she’d been asleep up until this point. “Who are you?” she asks in a bell-like voice that rings through the darkness. “If you need something of the parish, I’m afraid you’re going to have to come back in the morning when we’re open.” She pauses to think. “Well, if it's something simple like food, then I suppose I could find something for you in the kitch—”

Byleth cuts her off by leaning in and claiming her prey’s lips for her own. To her surprise, she finds little resistance. The woman’s eyes fly open wide, and as she begins to say something, Byleth takes the opportunity to slip her tongue into the woman’s mouth. After a few moments of taking her due, Byleth feels like the woman is pliable enough for her to make further suggestions.

“What is your name?” she asks, licking her lips as she pulls away. The woman beneath her stares up at her in awe, her mouth slightly agape as her chest heaves with each heavy breath. When the woman doesn’t immediately answer her, Byleth lets her hand wander down, cups one of the woman’s sizable breasts in her palm, and runs her thumb against the nipple that hides beneath a thin nightgown. “Your name?” she prompts once again, even as the woman gasps beneath her.

“Mercedes,” the woman admits breathlessly. “My name—it’s Mercedes.” While Byleth has never been one for worship, she can admit that her name sounds like a hymn to the very heavens above. With a wry smile of satisfaction, she leans back in to kiss her once again.

“Now’s not the time for altruism, Mercedes” she murmurs against the woman’s plush mouth. “Tonight, you are to tell me your deepest, darkest desires. And I shall fulfill them.”

“I must be dreaming,” Mercedes says heavily, brows furrowing as she turns her head to the side, away from Byleth’s ministry, and frowns. Not one to be dissuaded, Byleth just shifts her attention to dragging her tongue against the curve of her ear. Mercedes, holy as she is, tries to bat Byleth away as she continues speculating. “Perhaps this is a test from the Goddess.”

“Or a gift,” Byleth suggests, letting her voice rumble seductively. It’s a blatant lie, but she’s no angel. She doesn’t subscribe to the millions of pointless rules that they have, like ‘don’t lie to the humans’ or ‘don’t tear the humans apart from the inside out.’ No, she’s free to do as she likes—and now that she’s made contact with Mercedes, she decides that what she’d like is to absolutely wreck the sweet smile beneath her.

Mercedes seems relatively unfazed and, instead of squirming beneath Byleth in an attempt to escape, she looks up at her with those shining blue eyes. “I’m to tell you my deepest darkest desires, you say?” she asks, batting her long lashes coquettishly. “I’ll tell you,” she whispers, so pleasantly that if Byleth were either more foolish or prone to human emotions, she might have wished to imagine that they were lovers, “but first you have to tell me  _ your  _ name.”

Byleth lifts her brows as Mercedes’ request catches her completely off-guard. Most people merely fixate on their current object of affection while she’s with them. She doesn’t know how many times she’s heard people moan and scream the names of their beloveds. But to hear that this woman here wants to know who  _ she  _ is? It sends a shiver down her spine, and that shiver runs all the way to the very tip of her tail.

“You don’t need to—” she begins to say, but Mercedes’ dazzling smile just widens. The woman shakes her head.

“No. I want to know your name.” The gentle, yet utterly confident way in which she speaks makes Byleth begin to wonder just who between them holds the power. But she can’t deny that the thought of hearing her own name moaned while Mercedes was in the throes of ecstasy… it’s a pleasant one. And isn’t Byleth in the very business of pleasure? 

She smiles around her jagged teeth and, tucking a strand of hair behind her pointed ear, leans in to bestow her name upon Mercedes like the gift it is. “My name is Byleth,” she purrs before nibbling on the lobe of her ear.

“Byleth…” Mercedes repeats carefully, thoughtfully, almost as though it’s a name she should know. Yet Byleth pays her musing barely any mind, as she comes across the vein that trails down Mercedes’ neck and begins leaving little discolored marks all across the sensitive skin. A moment later, Mercedes gasps as Byleth hits upon a particularly dense nerve cluster. Her fingers tangle in the sheets below her, and Byleth feels a spark of pride.

As soon as she’s caught her breath, Mercedes speaks up from beneath her. “You want to know my desires?” she asks one more time in a husky voice, to which Byleth nods against her skin. Then, before Byleth has any chance to react, or even process what’s going on, Mercedes holds her close and flips the two of them over on the bed. Byleth gasps in shameful surprise as her back hits the mattress. She hurries to recover, only to look up and see Mercedes propped up above her, hunger in her eyes and want in her smile.

“My desire,” she whispers, leaning in close to Byleth’s ear in a gleeful mockery of how Byleth had been positioned just mere moments before, “is to see you writhing beneath me. My desire is to make you come so hard that you call out to the Goddess above in repentance for your sins and pray that maybe, just maybe, you may return to Her grace.” 

Byleth’s eyes widen as Mercedes perches over her stomach and runs her soft hands down Byleth’s body. From the slight, ribbed horns atop her head to her collarbone, from the swell of her breasts to the taut muscles of her stomach, Mercedes inspects every inch of pale, ash-grey skin available to her like she’s looking a peach over for bruises before sinking her teeth in. Byleth swallows at the thought of such a luscious nectar dripping down Mercedes’ neck, and she presses her thighs together in desperation as Mercedes teases with the hem of Byleth’s scant, admittedly almost non-existent shirt.

“For we are all Her creation,” Mercedes hums, peeling up the fabric so that from the waist up, Byleth is laid bare upon Mercedes’ sheets. She tosses the shirt aside gleefully. “And what better way to love Her, than to honor and glorify Her creation?”

Byleth’s breasts, free from the confines of her clothing, rise and fall with each of the demon’s labored breaths. Mercedes seems to take a new interest in them, and mimicking Byleth’s teasing from earlier, flicks one of her nipples lightly. The touch stings pleasantly, and Byleth hisses with arousal. Noting Byleth’s reaction, Mercedes’ already self-satisfied expression takes on an impish cast, and she leans forward.

Byleth arches her back and groans as Mercedes takes her nipple into her mouth. Her tongue, which had just moments before been praising the name of the Enemy Herself, licks and swirls against the sensitive skin inside like Mercedes expects some sort of reward for her effort. All she achieves, however, is a growing wetness between Byleth’s thighs that not even the demon herself can deny.

She wishes desperately that she could. It’s pitiful,  _ despicable _ of her, a great demon in her own right, to be so easily pushed and pulled by a single human woman, holy as she may be. She’s not sure how she intends to warp the situation in the eyes of her supervisor as soon as she returns home, but she isn’t given the chance to dwell upon it further. It seems as though Mercedes has other plans in mind.

The woman detaches from her breast with a tantalizing  _ pop _ and, with a beautiful smile, looks up at Byleth from under those thick, dark lashes she had admired so long ago. “Do you understand Her grace now, Byleth?” she asks teasingly, and Byleth pushes herself up on her elbows to better see her.

“It’ll never happen,” Byleth replies, her top lip curling in a snarl despite the way that the heat building in her core surges sharply at the sound of her name— _ her  _ name!—dropping from the plump, kissable lips of the woman sitting atop her. Mercedes merely clucks her tongue, however, and returns to lavishing attention upon Byleth’s other, more neglected breast.

This time, however, Mercedes takes it upon herself to reach down, her hand passing beneath her thighs, and press her fingers against the front of Byleth’s underwear. Byleth lets loose an unbidden groan at the unexpected stimulation of damp fabric against the heightened sensitivity of her clit, and she feels Mercedes smile around her breast. Without warning, her deft fingers push aside the fabric of her underwear and began to rub directly against Byleth in just the right spot to do the most damage.

Digging her fingers into Mercedes’ hair, with barely the presence of mind to keep her talon-like nails out of the human’s delicate scalp, Byleth keens. The thought of being so utterly  _ dominated  _ by a human is unbearable; yet even as she grinds against Mercedes’ hand in a desperate attempt to generate more friction, there’s a new intricacy to the act for which she was initially created. A new  _ intimacy _ , and as much as she hates Mercedes for taking control of her, she can’t thank her enough for opening her eyes.

Apparently satisfied with what she’s done with Byleth’s breasts, Mercedes shifts her attention elsewhere. Moving back on the bed, Mercedes meets Byleth’s eyes and, to her surprise, pulls Byleth’s soaked underwear off, spreads her legs apart, and dives in deep. Her tongue takes the place of her fingers, and the additional, all-enveloping warmth that hits Byleth’s clit then is almost too much for her to take. Mercedes’ hand, not to remain idle, slides down to play with the copious amount of lubricant that has built up at Byleth’s entrance. Every so often, a finger or two or three slip past her folds and hit a spot that makes stars appear in Byleth’s vision.

“Mercedes, Mercedes, don’t stop,” Byleth gasps, her head thrown back against the pillow Mercedes draws intricate spirals on her clit, as she toys with that magical spot, as pressure builds and builds and builds—

Her vision explodes as every raw nerve in her body is ignited with ecstasy, and Byleth falls back onto Mercedes’ bed with shaking limbs and breath that she can’t seem to catch. At some point, Mercedes pulls up from her position at Byleth’s core and, with a lazy smile that only accentuates the glistening of her lips and chin, says, “Praise be to the Goddess above, Byleth.”

“I can’t,” Byleth says in reply, her voice choking as she throws an arm up above her eyes to block her vision. “It’s impossible. You know what I am.”

“You’re a child of the Goddess,” Mercedes affirms smoothly, steadily, like she  _ hasn’t  _ just spent the last few minutes bringing a succubus to what is ostensibly the best orgasm of her life. Byleth can’t see her, but she can feel the mattress shifting beneath her as Mercedes repositions herself, pulls herself up high enough on the bed that she can run her fingers comfortingly through Byleth’s dark hair. To Byleth’s shame, her touch is a balm to her aching soul. “You may have fallen from the light long ago, but know that She has never given up on you.”

Byleth’s shoulders shake as she breathes heavily through the tears that threaten to overflow and spill down her face. She can’t look at Mercedes; she can’t imagine what kind, piteous look that she’d see if she did. “I’m not worth saving,” she forces through gritted teeth. “If I was, then She would have come for me a long, long time ago.”

She’ll never convince Mercedes to understand through simple words; her faith and conviction is too strong to shake. Yet at the same time, she can’t just  _ leave  _ Mercedes there. Her task would be incomplete, and Byleth can’t imagine what wrath she would face at the hands of the Big Guy for her failure.

Then, it strikes Byleth. Mercedes, beautiful and dominating as she is, is nothing more than a human. While Mercedes has indeed had the upper hand for so long, Byleth is a succubus. She was literally  _ made for  _ this; she has the power to turn things around.

“I can never be saved.” She grasps Mercedes’ upper arms and, holding her tight, twists the two of them around so that she’s finally back on top of Mercedes. The air leaves Mercedes’ chest in a surprised  _ huff,  _ and her sweet breath makes Byleth’s hair flutter around her face. Then, Byleth growls darkly. “But I can drag you down with me.”

She gives Mercedes no chance to catch her breath as she drags her sharp nails down the front of the woman’s nightshirt, shredding the fabric and revealing what lay beneath. Byleth eyes the smooth rolls of skin that Mercedes’ clothes had been hiding beneath the unnecessary fabrics. The temptation to bury her face in the softness of her belly is a difficult one to resist, though Byleth wouldn’t have bothered with resisting if she didn’t have a point to make.

Mercedes’ underwear soon follows the path her nightgown has taken, and Byleth positions herself between Mercedes’ spread thighs. Thinking quickly, she draws upon one of the more  _ interesting _ gifts she's been given as a demon of lust. She smiles with a sweet, sinful satisfaction at the shocked expression on Mercedes' face, when the mortal feels the tip of a brand-new appendage pressing at her entrance.

“That wasn’t there before, was it?” Byleth smirks, reveling at the efficacy with which she’d regained control of the situation. Mercedes’ lips form a beautiful ‘o’ as she registers what had just happened. “Is there anything else you’d like,  _ Mer-ce-des _ ?” Byleth draws out the woman’s name like a song, even as she tosses her head in a joyous motion.

With a deep breath and a shy smile, Mercedes shakes her head. To Byleth’s added pleasure, Mercedes then wiggles her hips to press herself further closer to the demon’s member. Delighted by the unspoken consent, Byleth lets herself sink into Mercedes’ already-slick entrance.

Mercedes’ eyes roll up to the ceiling, and her mouth begins moving with inaudible words. Byleth groans at what she has to assume is a prayer. A holy woman indeed. Moving beyond her initial disgust at having Her name brought into their exchange, Byleth sighs with delight as she settles deep into Mercedes, and remains still as she kindly waits for Mercedes to acclimate to her girth.

A moment later, Mercedes shifts around to test how she fits around Byleth’s dick, the motion bringing an unexpected gasp to Byleth’s lips, and a mischievous light shine in Mercedes’ bright blue eyes at the sound. “Didn’t I say that all living things are Her work?” she teases with a tiny wrinkle of her freckled nose. “That includes you, Byleth. Demon or not.”

With a long, languid sigh, she works herself down further upon Byleth, which elicits another moan from the demon, and lifts her arms up to wrap them gently around Byleth’s neck. The added pressure around her neck indicates that Mercedes is ready for more, so Byleth begins moving in and out. Her repetitive movement only takes a moment or two to build up to a steady rhythm fueled by a carnal need for  _ more. _

Mercedes moans as the pace picks up, as Byleth instinctively finds the perfect balance between friction and relief, and she tightens her grasp around Byleth’s neck so much that Byleth is pulled down to meet her waiting lips in an open-mouthed kiss. Despite the adjustment in position, Byleth continues thrusting her hips, pushing herself deeper and deeper with each movement in the hopes of driving Mercedes absolutely mad. But Mercedes simply makes happy, precious noises into Byleth’s mouth, and Byleth can’t tell whether the impossible heat blooming in her core is from the delicious pressure of Mercedes’ around her length, or if it’s from delight at knowing Mercedes is so obviously enjoying herself. 

Byleth realizes that she can be doing more. Even as she pants above Mercedes, desperately drinking in her kisses whenever she has the breath to do so, she lets one hand wander down to brush against Mercedes’ clit. Mercedes clenches tightly around Byleth at the introduction of the additional stimulus, and it’s only a moment before Mercedes begins to grind herself against Byleth’s length deep within her, against the hand that plays with the bundle of nerves set above her folds. 

Mercedes clutches at Byleth, her nails scrabbling across the demon’s back as she inhales sharply. She seems to be on the brink of orgasm, Byleth notes with pride. As though Mercedes is determined to prove her right, she cries out in a soft, high voice that makes Byleth think that maybe She was onto something in creating humanity after all. Or, well, in creating Mercedes.

Mercedes cries out again in just the same voice as her back arches up from the mattress. She tightens around Byleth in the most wonderful way possible, and Byleth finds that seeing the exhilaration that spreads across Mercedes’ face as she climaxes is enough to bring Byleth there to join her.

The experience of reaching orgasm alongside Mercedes feels akin to falling from the heavens itself: dangerous, heart-stopping, and seductive in its thrill. There is no risk of actually  _ procreating  _ with Mercedes, as anything Byleth produces is nothing more than pure magic, so she allows herself to fall apart while within the mortal.

As Mercedes finally relaxes beneath her, Byleth breaks the spell supplying her temporary appendage and collapses on the bed by her. With a chuckle, Mercedes reaches over and, once again, tenderly combs her fingers through Byleth’s blue hair. Her dreamers’ eyes crinkle at the corners. “Looks like there was something holy about you after all,” she whispers, “for that was divine.”

“I was supposed to corrupt you,” Byleth admits, to which Mercedes only laughs more. “I… don’t think I did a very good job.”

“Oh, any lesser woman would have absolutely fallen to your charms. I’m just special.” Byleth finds herself laughing alongside Mercedes at her outrageous claim. To be fair, there is indeed something special about Mercedes. But the other woman doesn’t seem to be done talking yet. “Will you stay with me? Just until I fall asleep.” 

The question catches Byleth off-guard. With a surprised blink, she nods. “Just until you fall asleep,” she confirms.

With an angelic smile, Mercedes nestles against Byleth’s side and closes her eyes.

Glancing at the dozing woman that’s using her arm and shoulder as a pillow, Byleth finds herself smiling. The passion that she’d found in their rendezvous—that Mercedes had supplied her with—had been unexpected, yes, but it had also been as far from unpleasant as it was going to get. 

She’ll have to tell her boss that corrupting Mercedes is taking a little longer than expected.

**Author's Note:**

> Well, that's my first time publishing smut. Hope it was alright! Lily wants more, though, so if anyone'd be interested in me playing around in this sandbox a little more, please let me know! 
> 
> Thank you to [Lily](LINK) for her endless help, sharp eyes in beta-ing, and the title. And additional thanks to [Fox,](https://archiveofourown.org/users/FoxVII/pseuds/FoxVII) [Quorn,](https://archiveofourown.org/users/brooklynapple/works) and [Rocky](https://archiveofourown.org/users/sambharsobs/works) for their inspiration ♡
> 
> [My twitter](https://www.twitter.com/tansybells) || [My tumblr](https://www.tansybells.tumblr.com)
> 
> Thanks for reading! Hope you enjoyed ♡


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